In an electrophotographic image forming device, an electrostatic latent image on a photoreceptor that is an image carrier is developed with the toner using a development device to form a toner image. The toner image on the photoreceptor is transferred onto a recording sheet, for example. In the transfer process of such an image forming device, the electrostatic transfer scheme is generally employed.
When a toner image is transferred onto a sheet that is the transferring body, voltage is applied by means of a transfer roller or the like from the backside of the sheet arranged to face the photoreceptor. An electric field is developed between the photoreceptor and the recording sheet, which causes the toner image to be electrostatically adsorbed onto the recording sheet.
Then, the transferred toner image is fixed on the recording sheet by a fixing device applying pressure.
For the development device of an image forming device, there are conventionally known a wet type development device and a dry type development device, both supplying a developer to a development roller that is a resilient member through a transportation roller, allowing the electrostatic latent image on the photoreceptor to be developed by the development roller. In such a development device, the development roller that is a resilient member takes a stopped state when an image is not formed, i.e. in a non-image formation state.
A continuing state of the surface of the resilient member (development roller) partially pressed in contact with another member (for example, transportation roller) causes compressive strain at the contacting region on the surface of the resilient member. Since the contact pressure between the members will be degraded at the region where such compressive strain is generated, the developer cannot be delivered evenly when the resilient member begins to rotate again for image formation, leading to the problem of unevenness in the density by the rotation cycle of the resilient member.
The mechanism of density unevenness occurring by the strain will be described hereinafter based on a wet type development device.
In the case where the member at the delivering side of the liquid developer and the member at the receiving side of the liquid developer are rotating in the same direction at the facing region, the liquid developer will pass through the nip between the members to be distributed to each member generally in the proportion corresponding to the speed ratio of the members.
However, the event of a strain at the surface of the resilient member will probably cause more liquid developer to be carried at the region that is concave by the strain.
Therefore, in the case where there is a strain at the member of the delivering side and more developer is carried at that region, the transported amount on the member of the receiving side opposite to that certain region will be increased.
In the case where the member at the delivering side of the liquid developer and the member at the receiving side of the liquid developer are rotating in opposite directions at the facing region, the liquid developer on the member of the delivering receiving side will not pass through the nip between the members, and will be transported to the member of the receiving side prior to passage through the nip. However, if there is a strain at the member of the delivering side, the liquid developer will readily pass through the nip between the members since the contact pressure is reduced where such strain occurs. Therefore, the amount of the liquid developer transported onto the member of the receiving side will be reduced at the passing-through region.
By the above-described mechanism, the generation of a strain at the resilient member will increase or reduce the amount the liquid developer to be transported, leading to occurrence of uneven density corresponding to the rotating cycle of the resilient member with a strain.
Such a problem in uneven density occurs similarly in a dry type development device.
PTL 1 discloses the approach of, in a liquid development device including a development roller, a developer transportation roller supplying a liquid developer to the development roller, and a developer supply roller supplying the liquid developer to the developer transportation roller, reducing the circumferential speed difference of these rollers in a startup operation than in a developing operation in order to reduce the torque during startup. Furthermore, PTL 2 discloses the approach of shifting the position of the development roller and application roller in the thrust direction from an unstable state to a stable state (steady state) by executing a rotation driving operation of the development roller and application roller (a preliminary driving operation), prior to the timing of starting a developing operation.
The art disclosed in PTL 1 and PTL 2 proposes the scheme of rotating a development roller or the like at the time of starting the development device or prior to an image formation operation.